Retrovirus-Like Gene Families in Normal Cells: Potential for Affecting Cellular Gene Expression

DNAs of normal animal cells contain gene families that are distinguished by many properties which are also shared with retroviruses and transposable elements, but are, neverthelsess, unrelated to retroviruses (i.e. there is no nucleic acid homology with known viruses). It has recently been shown that cellular retrovirus-like elements may occasionally act as natural mutagens. In order to assess the potential of retrovirus-like elements to affect expession of cellular genes it is necessary to expose all resident retrovirus-like families, study their involvement in DNA-rearrangements and analyze their control elements. We addressed these issues in respect to a specific ‘retrovirus-like’ family, designated VL30, of which over one hundred copies are dispersed throughout the mouse genome. We have shown that VL30 elements contribute to the overall genomic fluidity through participation in recombinations. These recombinations may juxtapose cellular sequences next to VL30 LTRs and may also create new LTR structures from the pool of cellular LTR units. Certain resident LTR units possess transcriptional promotor and enhancer activities that are more efficient than those of a strongly transforming retrovirus. The notion that additional retrovirus-like families still await exposure has been demonstrated here by the exposure of a novel retrovirus-like gene family.